This application is based on French Patent Application No. 98/14492, filed on Nov. 18, 1998, which is incorporated by reference herein.
1. Field of the Invention
The invention relates to digital transmission systems and methods and more particularly a digital transmission method or mode which can be used for transmitting data between two items of equipment, for example between an electronic chip card and a reading terminal.
2. Related Background
There are two modes of transmission or digital communication between items of equipment, for example of the data processing type, one known as xe2x80x9csynchronousxe2x80x9d and the other known as xe2x80x9casynchronousxe2x80x9d.
In the case of synchronous transmission, the elementary data are placed side by side and transmitted without interruption at the rate of a transmission clock signal. The start and end of a set of elementary data are marked by a characteristic succession of logic state transitions which takes place at the rate of the clock signal. There is therefore, for each elementary data item, neither a synchronisation signal nor a check signal.
In the case of asynchronous transmission, the data are transmitted individually and contain the information to be transmitted, synchronisation signals and check signals.
The synchronisation signals normally consist of a start bit of the transmission element and one or two end bits of the unitary transmission element. The check signals normally consist of redundant bits associated with the elementary data item.
The time separating two consecutive elementary data items is variable.
For the same frequency of transmission of the data bits, asynchronous transmission is slower than synchronous transmission because of the presence of synchronisation and check signals and any dead time between two consecutive elementary data items. However, synchronous communication requires the transmission of data clock signals, signals which are not always available on the communication channels of data processing equipment.
Moreover, in the case of the asynchronous transmission chip card, the check signals which are currently used, parity check for example, present a risk of not detecting certain transmission errors liable to occur in the case of chip cards since it is possible that two errors of the same nature may compensate for each other.
One aim of the present invention is therefore to propose a digital transmission method which does not present the drawbacks of the digital transmission methods of the prior art, notably those used by the transmission of data between an electronic chip card and a reading terminal with which the card may be in relationship, with or without contact.
The invention relates to a method of transmitting data between an item of equipment referred to as xe2x80x9creferencexe2x80x9d equipment and an item of equipment referred to as xe2x80x9cfollowerxe2x80x9d equipment, such as a microcircuit card and a terminal for reading/writing to the said card, characterised in that it comprises the following steps consisting in:
a) grouping together, in transmitting equipment, the elementary data of a message to be transmitted in the form of a series of at least one elementary data item consisting of at least one series of elementary digital signals,
b) transmitting the said series of elementary data to receiving computer equipment whilst causing it to be preceded by synchronisation signals which constitute, with the series of elementary data, a frame, the characteristics of duration and position of the elementary digital signals of each elementary data item and of the synchronisation signals being specific to the reference equipment.
The method defined above can be supplemented by a step (c) consisting of:
transmitting, to the receiving equipment, check signals for the transmitted data which consist of elementary digital signals.
The method is also designed to interrupt the transmission of the elementary digital signals for an interval of time separating two consecutive elementary data items, the said interval of time having a duration which is specific to the reference equipment.
This interval of time or window can be used for transmitting, during the said interval of time, at least one signalling signal indicating the end of transmission of a series of elementary data items, the said signal signalling the end of transmission of a series of elementary data items having characteristics of duration and position in the interval of time which are specific to the reference equipment.
The interval of time or window can be used to transmit, from the receiving equipment to the sending computer equipment, a signalling signal, referred as a data flow control signal, the said data flow control signal having characteristics of duration and position in the said interval of time which are specific to the reference equipment.
The number of elementary data items which are grouped together for transmission is variable from one frame to the next.
Each of the elementary digital signals of a data item has a duration which is specific to the reference equipment.
The elementary digital signals of a data item have relative positions with respect to each other which are specific to the reference equipment.